Fee picker in "Mint NFT" can be unset

[sk33z3r]

System:

• NixOS 21.11
• Firefox 97
• Brave 1.35.101
• Android 11
• Loopring Wallet 2.1.3.179

Repro:

1. Use WalletConnect on loopring.io to connect and unlock your wallet
2. Go to L2 Wallet > My NFT
3. Click on "Mint NFT" button in top-right
   OBSERVE: "Mint NFT" modal pop up appears
4. Click the Fee drop down
   OBSERVE: More fee token options appear
5. Click on the highlighted button (usually LRC)

Expected:
Nothing happens, fee amount and highlighted token remain unchanged

Actual:
The token is unchecked, the fee amount changes to -- undefined, and an error shown about insufficient funds

Notes:

• Minor issue considering it doesn't cause any disruption, and blocks the user from trying to mint due to no fee selected. I feel the UI would be cleaner if the user simply wasn't able to deselect all tokens since one must be set to move forward. The -- undefined part throws me off, seems like a bit of backend leaking out to the frontend users.